A federal judge yesterday barred prosecutors from pursuing the death penalty against a Jordanian man accused of leading a deadly plane hijacking in Pakistan 17 years ago, ruling that death penalty statutes established after the attack cannot be applied retroactively.

U.S. District Judge Emmet G. Sullivan in Washington ruled that prosecutors cannot seek the death penalty for Zayd Hassan Safarini, an alleged member of the Abu Nidal terrorist organization, who spent 14 years in a Pakistani prison after he was convicted in that country of hijacking Pan Am Flight 73 in 1986. Two Americans were killed in a 16-hour standoff on the tarmac in Karachi when the gunmen opened fire and tossed grenades at passengers.

Federal agents, who arrested Safarini last year after he was released from prison in Pakistan, brought him to Washington, where he faces a 95-count indictment on U.S. charges, including five felonies that could bring the death penalty.

But Sullivan ruled that the prosecutors' bid to use the Federal Death Penalty Act of 1994 to prosecute a crime that occurred eight years earlier is unconstitutional.

"In light of the strong presumption against the retroactive application of statutes, and in the absence of clear congressional intent to the contrary, the court cannot find that the Federal Death Penalty Act of 1994 applies to homicides . . . committed prior to its enactment," he concluded in a 27-page opinion.

Channing Phillips, a spokesman for the U.S. Attorney Roscoe C. Howard Jr., said prosecutors had not decided if they would appeal the ruling. Robert L. Tucker, the federal public defender who represents Safarini, said: "Obviously, we are pleased."

Safarini is accused of being the ringleader of a group of four men who stormed the aircraft on Sept. 5, 1986, as it waited to take off for New York with 379 passengers on board. The pilots escaped during the takeover, leaving the terrorists stranded with the passengers aboard the plane.

Safarini is accused of giving the order to other gunmen to open fire and toss grenades into a crowd of passengers at the center of the plane. Twenty-two people were killed and more than 100 were wounded before Pakistani security forces stormed the plane, arresting all four hijackers.